Frequently Asked Questions

What is AIS?

The Universal Shipborne Automatic Identification System (AIS) is a system of transponders installed on vessels which transmit over two dedicated digital marine VHF channels. The data is sent from each vessel every few seconds and contains position and movement information such as course, speed, latitude, longitude, and rate of turn. Static information about the vessel is sent every few minutes and includes the name, type of ship, length, beam, draft, etc. Nearly all commercial ships are mandated to carry AIS transponders and each year more yachts, launches and work boats are fitting AIS equipment.

Who uses AIS?

Every year, more and more vessels are voluntarily installing AIS equipment due to the enormous safety benefits. During the 73rd Session of the IMO's Maritime Safety Committee, the following vessels were identified as requiring AIS equipment as a mandatory requirement.

All ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size shall be fitted with AIS, as follows:

  • Ships constructed on or after 1 July 2002
  • Ships engaged on international voyages constructed before 1 July 2002
  • In the case of passenger ships, not later than 1 July 2003
  • In the case of tankers, not later than the first "safety equipment survey" after 1 July 2003
  • In the case of ships, other than passenger ships and tankers, of 50,000 gross tonnage and upwards, not later than 1 July 2004
  • In the case of ships, other than passenger ships and tankers, of 10,000 gross tonnage and upwards but less than 50,000 gross tonnage, not later than 1 July 2005
  • In the case of ships, other than passenger ships and tankers, of 3,000 gross tonnage and upwards but less than 10,000 gross tonnage, not later than 1 July 2006
  • In the case of ships, other than passenger ships and tankers, of 300 gross tonnage and upwards but less than 3,000 gross tonnage, not later than 1 July 2007

The United States Coast Guard also requires AIS on certain vessels not subject to SOLAS under new USCG rules which take effect March 2016.

What is the difference between types of AIS equipment? ?

AIS Transponders: An AIS transponder receives and translates the AIS digital radio signals. It also sends AIS signals, making your vessel visible to others with AIS equipment. There are two types of transponders, Class A intended for SOLAS vessels and Class B for fitting on non-SOLAS category vessels. AIS transponders are sometimes referred to as transceivers in the marine industry.

AIS Receivers: An AIS receiver operates in the same way as transponder but is a receive only device. If your vessel has a receiver you are able to view vessels in your area but other vessels will not see you on their AIS equipment.

AIS Display: An AIS display interfaces with either a transponder or receiver to display the AIS radio signals in a useable format. A dedicated AIS display is very useful to allow critical safety information to always be available at a glance and can include features that aren’t normally found in plotters or computer programs. If the transponder or receiver is WiFi enabled, you are able to use your smart phone, tablet or computer as an AIS display.

AIS Aerial/Splitter: A dedicated AIS aerial is used to pick up and transmit AIS signals only. A AIS/VHF splitter can be installed in order to share an existing VHF aerial between your VHF radio and AIS transponder or receiver.

Can I connect AIS equipment to my computer?

Yes. You can take the output from all models of WatchMate and XB Transponder and send it directly to your computer's serial port. If you have charting or navigation software that accepts AIS data then you can run your computer at those times when you wish and still have the benefits of the AISWatchMate.

All models of WatchMate and XB Transponder also provide NMEA multiplexing which means they will merge both GPS and AIS data into a single output stream. They will also merge any NMEA-0183 instrument data provided to it, such as from a heading sensor, into the output stream. This allows you to combine multiple sources of NMEA data into a single feed for your computer or other devices.

AISWatchMate 850, WatchMate Vision , XB-6000 and XB-8000 can also plug directly into your computer's USB port, making interfacing even easier.

Can I use the equipment with a chart plotter?

Yes. If you have a chart plotter that accepts AIS data, you can hook it up directly to a WatchMate or XB Transponder as shown in the diagram below.

WatchMate as a Multiplexer

Should I use a splitter or a dedicated VHF antenna?

There are trade-off's between having a dedicated AIS antenna and sharing one with your existing VHF system through a splitter. A dedicated AIS-tuned antenna is an excellent choice and will typically out-perform a standard VHF antenna due to being tuned specifically for the high end of the VHF band where AIS is located. It can also serve as a backup antenna for your VHF. However, a dedicated antenna may be harder to install since you need to mount it and run the cable, etc. You should place it as high as reasonably possible but as far away from masts and other antennas, particularly other VHF antennas.

A splitter allows you to use your current VHF antenna instead of fitting another one which will generally lend itself to an easier installation. You'll probably get the best performance out of your AIS system this way too because your existing antenna is likely to be mounted up high. If you decide to get a splitter, you must use one designed for AIS. If you have a transponder you must use a splitter designed specifically for use with AIS transponders.

However, your VHF antenna may not be well tuned at AIS frequencies which will lead to poor transmit performance. You can check your antenna performance at AIS frequencies by looking at the VSWR curves provided by the antenna manufacturer (they can sometimes be found on the manufacturers website). Look for a VSWR of 2:1 or less at 162MHz. Most VHF antennas are well tuned for 156MHz but will start to degrade at the extremes of the VHF band.

Using a splitter also means you can't transmit on your VHF and receive AIS updates at the same time. In practice this usually isn't a big deal since on many boats transmitting on the VHF radio is limited. The AISWatchMate 850 or WatchMate Vision coupled with the Vesper Marine Antenna Splitter (SP160) gives you an indication on the AISWatchMate screen when the VHF radio is being used. This will also indicate if your MIC or VHF radio is stuck.

Can I use my existing VHF antenna?

Yes. An antenna splitter allows you to use your current VHF antenna instead of fitting another one which will generally lend itself to an easier installation. There are trade off's between using a splitter and having a dedicated antenna for your AIS system, read more about that in the previous FAQ.

If you decide to use a splitter, be sure to get one designed for use with your AIS system. A receive only splitter differs in functionality from a splitter that also allows you to transmit on the AIS path. Damage to either your VHF or your AIS equipment may occur if you use a splitter not well suited to your AIS system. If you have a transponder you must use a splitter designed specifically for use with AIS transponders, like our SP160 Antenna Splitter. Our splitter can also be used with AIS receivers such as the AISWatchMate 750 Receiver.

What kinds of vessels will be visible to me?

AIS transponders are required on all ships over 300 tons when operating internationally and 500 tons when operating domestically. Passenger ships are required regardless of their destination. This means that nearly every commercial vessel will have one. In addition, many yachts and other vessels voluntarily carry an AIS transponder. Each year more and more vessels are equipped with AIS because of the significant safety benefits.

How often is AIS information received?

AIS data is transmitted from vessels at a variety of rates. Class A ships send position updates every 2 - 10 seconds depending on their speed and rate of turn, or every 3 minutes when reporting as anchored/moored and not moving faster than 3 knots. Class B ships send position reports every 30 seconds when moving faster than 2 knots, otherwise it drops back to every 3 minutes. Information that doesn't change frequently, such as the vessel's name, size and voyage information, is sent every 6 minutes for both Class A and B.

The WatchMate continuously updates bearing, range, CPA, and TCPA for every vessel whenever a new position is received from the vessel and also whenever your vessel moves. In addition, the WatchMate displays the time that has elapsed since each vessel has sent its last update. This is useful to help you determine the currency and accuracy of the data presented.

How often is my AIS information sent from a transponder?

The AISWatchMate 850, WatchMate Vision, XB-6000 and XB-8000 are Class B devices so it will send position reports every 30 seconds when you are moving faster than 2 knots, otherwise it drops back to every 3 minutes. Other information such as your vessel's name, size and type, is sent every 6 minutes.

The WatchMate will display a warning symbol if your transmissions are not being sent for any reason. The XB Transponders status LED will change color to orange. You can also query the reason for the status change using the vmAIS Configuration and Status Utility or WatchMate App over USB or WiFi.

Installation Questions

Can I use my existing GPS receiver with an XB-6000, XB-8000, WatchMate 850 or Vision?

No. As a requirement of compliance with the international Class B AIS Transponder standards, the AISWatchMate 850, WatchMate Vision, XB-6000 and XB-8000 must utilize its own built-in source of GPS data. It will not process GPS data sent to it on the NMEA 0183 or NMEA 2000* ports. This regulatory requirement exists to ensure that GPS data cannot be inadvertently switched off and ensures the GPS receiver meets the strict AIS performance standards for accuracy and immunity to interference.

However, each of our transponders can be used as a primary or backup source of GPS data for other equipment on your vessel. All AIS and GPS data it receives is output on its USB, NMEA 0183, NMEA 2000 and WiFi* ports (when configured to do so, *Vision and XB-8000 only).

How do I connect an external alarm or buzzer?

You can easily connect a 12 volt buzzer or indicator to the external alarm port of the WatchMate. The AISWatchMate 670 and 750 can switch any device that draws up to 2 amps without the need for an external relay on its external alarm port.The AISWatchMate 850 and WatchMate Vision provide a control signal on the external alarm port, designed to drive a low power buzzer or indicator or an external switch or relay. The wiring diagrams for each model show how to connect an external alarm, buzzer or indicator. Vesper Marine has designed a unique external alarm that is suitable for both 12 and 24 volt boats and reverse polarity protected.

How will a transponder integrate with NMEA information?

All models of WatchMate and XB Transponders provide NMEA multiplexing which means they will merge both GPS and AIS data into a single output stream. They will also merge any NMEA-0183 instrument data provided to it, such as from a heading sensor, into the output stream. This allows you to combine multiple sources of NMEA data into a single feed for your computer or other devices as shown in the diagram below.

WatchMate as a Multiplexer

I have a Class B transponder. Can I connect the AISWatchMate 670 Display-Only to the NMEA output of the transponder and use the computer output from the transponder at the same time?

Yes you can. If your transponder has a cable with both the computer serial output connector (a 9 pin connector called a DB9) then you can plug it directly into a serial port on your computer or use a USB-to-serial adapter. You can then use the NMEA output from the transponder (for most Class B transponders these are the orange and black wires from the 4-wire gray cable - check your transponder manual to be sure). Select which port you want to use on the AISWatchMate and connect the orange to the data input for that port. Connect the black wire from the transponder to the signal ground for the same port.

When you hook it up this way, the data from the transponder is sent in parallel to both your computer and your AISWatchMate 670.

Can the WatchMate be hooked up to a Laptop and a NMEA device, such as a chartplotter, at the same time?

Yes, that's how it's designed to be used.

Can I share my existing GPS antenna with a transponder?

No. Most marine GPS antennas are active antennas, which means they will have a built-in LNA (low noise amplifier) to improve the signal to noise ratio of the incoming GPS signal. This LNA requires power which is obtained from a DC source on the device it is connected to. By connecting it to two devices that are each attempting to supply power you run the risk of causing damage to one or all of the devices. There are other complications that will inevtiablly arise too (e.g. noise introduction, signal loss) so it is highly recommended to avoid this approach.

The AISWatchMate 850, WatchMate Vision and XB-6000 have an internal GPS antenna built in. So if your installation will give the WatchMate a clear view of the sky then you may not need an external antenna at all. If you need an external GPS antenna, ensure compatibility and maximum performance by using a Vesper Marine External GPS antenna. The use of any other antenna is not recommended.

How do I connect my GPS antenna to my WatchMate 750?

The AISWatchMate 750 Receiver does not have a built-in GPS receiver, so there is no connection supplied for an external GPS antenna. The AISWatchMate 750 is designed to work with a stand alone GPS Sensor or can use a data feed from another GPS device on your vessel. The output from the GPS Sensor, or from another GPS device, is connected to the NMEA input port of the AISWatchMate 750.

I need to extend the length of the DC power cable supplied with my WatchMate or XB Transponder. What gauge wire should I use?

It depends on the length that you need to run, but for any length up to 25m (80') we recommend no smaller than AWG 16 (1.5mm2). Remember to always install a 2A fuse or circuit breaker between the WatchMate or XB Transponder and your battery or distribution panel. Please see our wiring diagrams for more information.

Troubleshooting Questions

All screens have "No GPS Position!"

If you have an AISWatchMate 670, be sure you have a GPS connected either directly to a port on the AISWatchMate or routed through an AIS receiver. If you are using a transponder, make sure the transponder is setup to provide GPS data. You can verify that GPS data is being properly received by the AISWatchMate by using the procedure for examining the incoming data stream.

Make sure you've got the port that the GPS is connected setup properly. Use the device settings menu to verify that "GPS" is selected as the input device. However, if the GPS data is being provided by an AIS receiver or transponder then select "AIS" as the input device.

Ensure the GPS has a valid satellite fix. If it doesn't indicate a valid fix then the WatchMate won't use the data it provides. Use the GPS Satellite Status page. You can also verify this by examining the GPS screen or by looking at the data stream on a AISWatchMate 670 or 750. When you are looking at the NMEA data stream, locate the $GPRMC sentence and you should see something that looks like this:


Look for the "A" in the 3rd field. If it is a "V", or something other than "A", then the GPS doesn't have a valid fix. Check for a loose antenna connection or other wiring problem on the GPS.

I always see headings and bearings in true rather than magnetic

This information applies only to a AISWatchMate 650. All other display models have built-in magnetic variation computations.

Check the setting "Course and heading mode" or "Bearing to target mode". If it is set to magnetic and you are still seeing your heading or target bearings in true, then it's because your GPS is not providing magnetic variation. Marine GPS units usually include this data in their RMC sentence, but embedded GPS modules, such as those used in transponders and other devices, do not provide magnetic variation.

If you have a transponder and you want magnetic headings or bearings then you will need to connect the AISWatchMate to another GPS. You can do this by using the second port on the AISWatchMate and selecting GPS as the input device for that port.

How can I examine the incoming NMEA data stream to check for problems?

It's sometimes useful to verify that either AIS or GPS data is being properly received by the WatchMate.

To do this on a AISWatchMate 850, 750 or 670, first turn it off. Then switch it back on by pressing the power button while holding the top button at the same time. At the top of the screen will be "Data Received from Port 1". The raw NMEA data stream will be displayed on the screen. You can pause the display by pressing and holding any button. When you release the button the data display will continue. If you see all dots (".") then it probably means you have the port set to the wrong device type or a wiring problem. Restart the AISWatchMate normally and check the settings for that port.

To exit from the raw data display, press the power button. To view the data stream on port 2, turn the AISWatchMate on by holding the 2nd button from the top while pressing the power button.

You can also look at the NMEA 0183 data stream on all WatchMate and XB Transponder products over USB or WiFi using the vmAIS Configuration and Status Utility

Why is my WatchMate 850 Transponder reporting Error 04 Antenna Problem and switching itself into silent mode?

The AISWatchMate 850 Transponder will report the error you are seeing if it senses the VSWR of the antenna connection is too high. This is done to ensure that most of the power that is being generated by the AISWatchMate 850 on a transmission is delivered to the antenna, and not just reflected back and lost.

A high VSWR typically indicates poor power transfer to the antenna. Practically speaking, this will mean your effective transmission range is reduced significantly, so it is important to get it right.

A high VSWR can be caused by a bad cable, connector or a poorly tuned antenna. The VSWR response of an antenna is frequency specific, so although your VHF antenna may work well at 156MHz, it may not be well tuned to the AIS channels at the upper end of the VHF band (162MHz).

Things to check:

  • Are the cables and connectors connecting your antenna to the AISWatchMate in good working order, connected tightly and corrosion free?
  • What is the specified VSWR of your antenna at 162MHz (information generally available from the antenna manufacturer)? A VSWR of 3:1 or more at 162MHz is not recommended for use with the AISWatchMate 850.
  • Antenna mounting arrangement. Is it mounted close to a metalic object? Are there any obstructions around the antenna pole? Does the antenna require a grounding connection to stay tuned? Is the antenna positioned at least 3 meters from the AISWatchMate?
  • Does your antenna present a DC open or short (normally stated in the datasheet)? Can you verify this with a multimeter at the AISWatchMate VHF connector end?

What does VSWR mean and why do I care?

VSWR stands for Voltage Standing Wave Ratio and is a measure of the how well matched a transmission line (e.g. your antenna cable connected to the VHF output of your WatchMate) is to its termination impedance (e.g. your antenna). VSWR can be thought of as a comparative measure of how much power, generated from your WatchMate or XB Transponder, is delivered to the antenna and how much of it is reflected back and lost in the poorly matched transmission line. You care about this as it will effect your transmission performance. A high VSWR will reduce your effective transmission range and can also lead to other problems. If your WatchMate is reporting a problem with your antenna connection or high VSWR, please look at the previous FAQ for troubleshooting.

To learn more about VSWR there is a great explanation given at For those not technically inclined all you have to concern yourself with is ensuring that the cables and connections from your WatchMate to your antenna are sound and your antenna is well matched at AIS frequencies (162MHz) and is not mounted too close to any large metal objects.

Usage Questions

What apps can I use on iPad, iPhone and Android with the XB-8000 and WatchMate Vision?

WatchMate Vision and XB-8000 are supported by a growing set of mobile apps. Things to look for are whether the app supports GPS and AIS data over WiFi (TCP). If so, then it will work great with either the WatchMate Vision or XB-8000. Here's a summary of apps we have tested.

How do I change the WatchMate Display Brightness Setting?

A quick way to adjust the brightness of your AISWatchMate screen is done by pressing the power button momentarily, a brightness adjustment display will pop up. From here just set the brightness as required and press OK, you are done.

What can I do if I'm hearing too many alarms?

If you are having too many alarms go off check your Profile settings. Adjust the CPA alarm distance and TCPA (Time to CPA) and check the Target Speed Alarm. You can use that setting to eliminate alarms for stationary vessels.

Do the settings in the WatchMate 850 Transponder setup control both the USB and NMEA output?

No. The settings only affect the NMEA ports. The USB port on the AISWatchMate 850 is a virtual COM port and always runs at 38,400 baud. All AIS and GPS data received by the AISWatchMate 850 is sent to this port.

When selecting port output of NMEA+AIS does the WatchMate GPS data get output with AIS data?

Yes. Also any incoming NMEA data received over the NMEA input port is multiplexed in.